The present invention relates to a magnetic body of metallic composite material of high performance level for use in choke coils and the like devices; more specifically, a composite magnetic body for use as a soft magnetic material of a magnetic core.
In line with the prevailing trend of down-sizing among the recent electric and electronic equipment, demand for a smaller but efficient magnetic body is increasing. For example, in many of the choke coils at high frequency circuit, a ferrite magnetic core formed of a soft magnetic ferrite and a dust core compression-formed of soft magnetic metal powder are used.
Of them, the ferrite magnetic core is noted for its defect of a small saturation magnetic flux density. In order to suppress the decline of the inductance L value and assure the direct-current superposing characteristic, a gap of several hundred microns is provided in a direction vertical to the magnetic path. Such a wide gap, however, may be a source of beat sound, or when used in a high frequency band, in particular, the leakage flux generated in the gap may extremely increase the copper loss in the winding.
By contrast, the dust core fabricated by forming the magnetic metal powder has an extremely large saturation magnetic flux density as compared with the soft magnetic ferrite, therefore it is advantageous in reducing the size of a core down. Also, generation of the beat sound and the copper loss caused by leakage magnetic flux are small, since it can be used without providing the gap.
However, in terms of the magnetic permeability and the electric power loss, dust core is not superior to the ferrite core. When a dust core is used in a choke coil or an inductor, it results in a greater temperature rise corresponding to the greater core loss; so it is hard to make the core size smaller. In order to improve the magnetic characteristics, the dust cores are usually formed by applying a compression force higher than 5 tons/cm2 or even more than 10 tons/cm2, depending on the kind of application. Therefore, it is quite difficult to form a dust core in a compact and complicated core shape; for example, a core for a low profile choke coil for use in a computer DC-DC converter. Thus, there is a greater limitation in the shape of the dust cores, as compared with the case of the ferrite cores. Down-sizing is not easy with the dust cores.
The core loss with the dust cores normally consists of hysteresis loss and eddy current loss. The eddy current loss increases in proportion to the square of frequency and the square of a flowing size of eddy current. Therefore, to suppress the generation of eddy current, surface of the magnetic powder is covered with an electric insulating resin or the like material.
Since the dust cores are formed with a high compression force, the magnetic permeability is deteriorated by the distortion caused in the magnetic body, which brings about a hysteresis loss. To avoid this to happen, a high temperature heat treatment is applied on the compression-formed pieces for relieving from the distortion. In such a case, the use of an insulating binder is essential to ensure a good insulation between the magnetic powder particles while keeping good mutual adhesion.
A conventional magnetic dust core is disclosed in Japanese Laid-open Patent No. 1-215902, which core is formed of a mixture of a magnetic alloy powder, Fexe2x80x94Alxe2x80x94Si alloy (sendust) or Fexe2x80x94Ni alloy (permalloy), and an alumina cement powder, which mixture is compression-molded after annealing at 700-1200xc2x0 C. Japanese Laid-open Patent No. 6-342714 teaches a magnetic dust core which is formed of a mixture of an Fexe2x80x94Alxe2x80x94Si alloy magnetic powder and a silicone resin, which mixture is compression-molded and then annealed in non-oxidizing atmosphere of 700-1200xc2x0 C. Further, Japanese Laid-open Patent No. 8-45724 discloses a magnetic dust core that is formed of a mixture of an Fexe2x80x94P alloy magnetic powder, a silicone resin and an organic titanium, which mixture is compression-molded and then annealed in 700-1200xc2x0 C. atmosphere.
With the ferrite core having a gap, the inductance L value declines suddenly from a certain point in the direct-current superposing current. In the dust core, by contrast, it declines smoothly along with the direct-current superposing current, but the core can comply with a large current because of the high saturation magnetic flux density. For implementing a high magnetic permeability with the dust core, it is effective to increase the packing rate of alloy powder in a core piece and to reduce the distance between the powder particles.
However, increasing the packing rate contradicts to securing the insulation between the particles; as a result, it is difficult to realize the both requirements at a same time, the high packing rate and the insulation between the powder particles. Furthermore, it is difficult for a dust core to take a complicated shape, which means that there is a substantial restriction in the core shapes available.
The present invention addresses the above described problems existed in the conventional magnetic cores, and it is an objective to offer a composite magnetic body that satisfies both of the requirements at the same time, the high magnetic permeability and the small core loss. Furthermore, the composite magnetic body of the present invention can be formed into a core piece whose shape is of high complexity.
One mode of a composite magnetic body in accordance with the present invention is that which is formed of a mixture of a magnetic alloy powder containing iron (Fe) and nickel (Ni) as the main components and a silicone resin binder for binding the powder particles together, and the mixture is compression-molded. The alloy powder containing iron and nickel as the main components exhibits a high magnetic flux density, and admits a substantial plastic deformation during compression-molding process attaining a high packing rate of the powder in the compressed compact; thus it provides a high magnetic permeability. By combining the magnetic powder with a silicone resin working as a binder, insulation between the alloy powder particles after the compression molding is secured, and the eddy current loss is reduced; hence, a low core loss is realized.
Another mode of a composite magnetic body in accordance with the present invention is that which is formed of a mixture of a magnetic alloy powder containing iron and nickel as the main components, an insulating material and an acrylic resin binder for binding these together, and the mixture is compression-molded. The composite magnetic body in the present mode exhibits, like the magnetic body in the earlier described mode, a high magnetic permeability, and the insulating material assures good insulation between the alloy powder particles after the compression molding to a decreased eddy current loss; hence, a low core loss. The acrylic resin provided as a binder improves the compactibility, which contributes to the formation of a core having a high complexity shape
Still other mode of a composite magnetic body in accordance with the present invention is that which is formed of a mixture of an iron powder, or a magnetic alloy powder containing silicon of not more than 7.5% by weight (not including 0%) and iron for the rest, an insulating material and an acrylic resin binder for binding these together, and the mixture is compression-molded. Also in the present mode, the magnetic body exhibits both a high magnetic permeability and a low core loss, and the acrylic resin used as a binder improves the compactibility, which contributes to the formation of a core having a high complexity shape.